Remove unused and obsolete classes and tidy up:

Many classes required to support type-erasure of handlers and boost::asio
types are now obsolete, so these classes and files are removed:
HTTPClientType, FixedInputBuffer, PeerRole, socket_wrapper,
client_session, basic_url, abstract_socket, buffer_sequence, memory_buffer,
enable_wait_for_async, shared_handler, wrap_handler, streambuf,
ContentBodyBuffer, SSLContext, completion-handler based handshake detectors.
These structural changes are made:
* Some missing includes added to headers
* asio module directory flattened

beast/module/core/diagnostic/FatalError.h

@@ -20,6 +20,8 @@
 #ifndef BEAST_CORE_FATALERROR_H_INCLUDED
 #define BEAST_CORE_FATALERROR_H_INCLUDED
 
+#include <beast/strings/String.h>
+
 namespace beast
 {
 
@@ -39,7 +41,7 @@ class FatalError
 public:
     struct Reporter
     {
-        virtual ~Reporter () { }
+        virtual ~Reporter() = default;
 
         /** Called when a fatal error is raised.